Phase-selective variable-frequency oscillatory system



March 23, 194s. E, E, WHEELER 2,438,197

PHASE-SELECT-IVF VARIABLELFREQUENCY OSQILLATORY SYS'EI Filed Dec. 31, 1942 2 Sheet's-Sheot 1 E .AIII

z a N lm'lEm'oR nARoLoj A. WHEELER ma? oRNExx- H. A. WHEELER 2,438,197

Filed nec. 31. 1942 2 sums-snaai 2 INVENTOR 0 D ANI Yo. |||V Nm PHASE-SELECTIVE VARIABLE-FREQUENCY. OSCIILATRY SYSTH March 23, 194s.

mmoz. 7. mysw Il w mzlmm SU mv.. m M m f L V.Patented Mar. 23, 1948 PHASE-SELECTIVE VARIABLE-FREQUENCY OSCILLATORY SYSTEM Harold A. Wheeler, Great Neck, N. Y., assignor, by mesnc assignments. to Hazeltine Research, Inc., Chicago, Ill., a corporation of Illinois 'Application December 31, 1942, Serial No. 470,786

(Cl. Z50-36) 19 claims. 1

The present invention relates to electrical indicating systems for indicating unbalanced components of coupling in a normally-balanced system. In greater particularity, the invention relates Vto electrical indicating systems of the type in which variations of a coupling effect in an impedance network of the system is utilized to produce an indication. While the invention is oi general utility, it is particularly suitable for use in a system for indicating the presence of a hidden mass of material of such nature as to have a different eect in unbalancing the system than does the surrounding substance. for example for locating metal objects buried a short distance underground, and will be described in that connection.

It is frequently desirable to provide an electrical system which is initially balanced, but which is sensitive to unbalance and provides a desired indication in response to some change of condition causing an unbalance. Certain systems of this nature contain reactance components and resistance components and an unbalance of the system may be caused by a change in the value of either type of component. In particular cases, only the unbalance of one type of component is of interest, and that of the other type of component should be ignored. In such case, it is desirable that the indicating system be "phaseselective; that is, that it provide the desired indication only in response to an unbalance of the component of interest. It is also desirable that an automatic rebalancing arrangement be provided in the system which will compensate for unbalance of the component which is not of interest without aiecting any unbalance of the component of interest.

An example of such electrical indicating systems is to be found in devices for detecting the presence of a hidden mass of material having an electrical characteristic which distinguishes it from the substance which surrounds it. In particular, such systems are used to locate bodies of ore or to detect metal objects such as pipes and electrical conductors which are buried in the ground. Advantage is taken of either the magnetic permeability or the electric conductivity of the mass of material to cause an unbalance of coupling between exposed coils, leading to an indication distinguishable from that caused by the electrical characteristics of the surrounding substance. Ordinary dry ground produces by its slight magnetic eiiect mainly a reactance component of unbalance in such systems at all audio-frequencies while salt water produces mainly a resistance component of unbalance at low audio-frequencies. If the system is phaseselective, the conductivity characteristic of a metal body may be utilized to locate such a body buried in dry ground, whereas the permeability characteristic may be utilized if the body is of magnetic material and is submerged in salt water.

The electrical indicating system of the copending application of Leslie F. Curtis. Serial No. 425,155, filed December 31, 1941, now abandoned and assigned to the same assignee as the present application, is of the phase-selective type. An oscillator is provided with a main feed-back path for producing sustained oscillations and an auxiliary feed-back path is provided for shifting the frequency of the oscillations in response to one phase of unbalance in the detecting system. The auxiliary feed-back path includes a pair of coils, having negligible vinductive coupling. and an amplifier. 'I'he auxiliary path is so phased relative to the main feed-back path that the system is normally balanced but becomes unbalanced if the coupling between'the coils is changed in a certain phase, as by a metallic body brought into the vicinity thereof. 0n becoming thus unbalanced. the coupling through the auxiliary path changes the frequency ofthe generated oscillations and suitable means is provided for observing the change of oscillator frequency to indicate the proximity oi the metallic body. The sensitivity of this arrangement is greatly enhanced by the use of an amplifier in the auxiliary feedback path for the purpose of amplifying the unbalance component which causes the change of oscillator frequency. The relative phases of coupling in the main and auxiliary feed-back paths must be maintained if the system is to be unresponsive to one component of unbalance. as for example the resistance component. and responsive to the other component, for example the reactance component. This requires considerable accuracy in the design and adjustment of thhampliier circuits in the auxiliary feed-back pa It is an object of the present invention to provide a new electrical indicating system of the type described, improved in performance and stability but simplified in construction.

It is a further object of the invention to provide a new and improved electrical indicating system of the type described which avoids the necessity for any critical relative phasing of aplurality of circuits to maintain its phase-selective properties.

It is an additional object of the invention to provide a new and improved electrical indicating system of the type described having a high degree of phase selection by which to detect .and indicate a small component o! unbalanee of a certain phase while being unresponsive to components of unbalance of quadrature phase.

It is a further object of the invention to provide an improved electrical indicating system of the type described which requires a minimum number oi' initial adjustments to place the system in an' operative condition of balance sensitive to unbalance.

In accordance with the invention, a variablefrequency oscillatory system comprises a stable amplifier having input and output terminals and having substantial amplification and a positive phase slope over a predetermined frequency range. The system includes a single feed-back path substantially nonselective over this frequency range coupling the amplifier output terminals to its input terminals, means included in the feed-back path for establishing a feed-back component oi' coupling of a type effective primarily to cause sustained oscillation of the amplifier at a frequency in the aforesaid range but ineffective to cause substantial variations of the oscillation frequency of the amplier with variations of the component. The system also includes means in the feed-back path for establishlng a second component of feed-back coupling substantially independent of and in quadrature relation to the first-mentioned component, the second component being variable in response to a frequency-control condition external to the last-mentioned means and being effective primarily to cause variations of the frequency of oscillation in accordanceV with variations of the condition.

In a particular form of the invention, the aforesaid components of coupling comprise, respectively` reactance and resistance components of coupling andthe aforesaid feed-back path includes a coupling impedance having two portions which individually provide the components of coupling. This coupling impedance comprises an auxiliary primary impedance providing primarily the aforesaid one component oi.' coupling and a pair of inductors normal-ly having substantially no mutual inductive coupling but becoming coupled to provide primarily the aforesaid other component of coupling in response to the aforementioned condition.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

Referring now to the drawings, Fig. 1 is a circuit diagram. partly schematic, of a complete electrical indicating system embodying the present invention; Fig. 2 is a graph representing the phase-slope characteristic of an amplifier included in the system: Fig. 3 is a graph representing the frequency-response characteristic of the amplifier; Fig. 4 is a vector diagram representing components of coupling occurring at certain points in the system; Fig. 5 is a circuit diagram of a portion of an electrical indicating system embodying a modified form of the invention; Fig. 6 is a circuit diagram, partly schematic, of a complete electrical indicating system embodying the present invention in an additionally modified form; and Fig. 7 represents a novel application of the invention.

Referring more particularly to Fig. 1 of the drawings, there is represented a complete elecl minals i l".

trical indicating system embodying the present invention in a preferred form. The system includes a stable amplifier I0 of one or more stages having input terminals ii and output terminals I2 and having substantial amplification and a positive phase slope over a predetermined frequency range which may be an audible-frequency range. The values of phase slope of the amplifier I0 preferably are substantially independent of the amplitude of oscillations of the amplifier. The optimum circuit design and circuit constants of amplifier I0 depend mainly on the operating frequency. If the operating frequency is to be an audio frequency, for example 1,000 cycles, the amplifier is much like a conventional audio' amplifier except that it preferably includes a single resonant circuit 9, which may include in parallel-connected relation an inductor 6, a resistor 1 and a condenser 8, very broadly tuned to 1,000 cycles. If the amplifier includes several stages, one stage should be broadly tuned as stated and the remaining stages preferably should have as little phase shift as possible over the range of response of the amplifier. Conventional ampliflers having these characteristics are described in Radio Engineering" by Terman, second edition, pages 261 and 262 (McGraw-Hill Book Company, New York and London). On the other hand, if the operating frequency is a radio frequency, for example 100 kilocycles, the amplifier is much like a conventional intermediate-frequency amplifier of a radio receiver. In this case, it is also preferable that the selectivity of the amplifier be provided by a single tuned circuit having a relatively broad resonance characteristic and that all other stages of a pluralstage amplifier provide relatively little selectivity or phase shift. Where the system is used to locate buried metallic bodies or bodies of ore in the ground, it has been found that lower operating frequencies provide greater depth of penetration in the ground and a frequency of 1,000 cycles has been found satisfactory for metallic objects buried several feet under the surface of the ground.

The indicating system also includes a single feed-back path including a coupling impedance I3, substantially nonselective over the operating frequency range of the amplifier i0, coupling the amplifier output terminals i2 to its input ter- I'his feed-back path has, by virtue of the coupling impedance I3, quadrature-related components of coupling of which one component, hereinafter called' the major component, is effective to cause sustained oscillations of the amplifier i0 at a frequency in the range of operation of the system, and the other component, hereinafter called the minor component, is variable in response to conditions to be indicated and is effective to cause variations of the frequency of oscillation in accordance with variations of the magnitude of the minor component 'I he reference impedance i4 comprises a pair of coupled inductors I8, I9 arranged as a transformer, the coupling between the lnductors being adjustable as indicated by the arrow. The inductor I8 may be considered the primary inductor of this transformer arrangement and is connected in series with inductor I6 in a primary circuit which is coupled to the output circuit terminals I2y of the amplifier I0. Similarly, the inductor I9 of impedance I4 may be considered the secondary of the transformer arrangement and is/,l0 connected in series with the inductor or sections `lh--IIt in what may be considered a secondary circuit which is coupled to the input circuit terminals Il ofthe amplifier I0. The design and construction of a preferred type of exploring unit I5 is described and claimed in the copending application of Harold A. Wheeler, Serial No. 470,785, filed concurrently herewith, and assigned to the same assignee as the instant application. This arrangement of the exploring unit I5 is one 20 in which the inductors I6 and Ila-Ils are supported` in concentric and coplanar relation as a unitary structure, the inductorsections Il. and I'Ib being of diierent diameters and connected with opposing fields and the inductor I6 being of 25 an intermediate diameter. This construction has numerous advantages, one of which is that the exploring unit I5 is adapted to be readily carried a few inches above the surface of the ground G in search of a buried metallic body M.

The indicating system of the invention additionally includes means responsive to the frequency of oscillation of the amplifier I0 for indieating variations of the minor component of coupling caused by variations of a condition to be indicated. This means comprises an oscillator 20, having a substantially constant frequency fu located at the center of the range of operation of the amplifier I0, and means for simultaneously audibly reproducing at substantially the same volume the oscillations of the amplifier I0 and those of oscillator 20 to indicate variations of the minor component of coupling caused by variations of the condition to be indicated. This lastnamed means comprises a beat-note indicator 2I 45 having a first input circuit coupled to the output circuit terminals I2 of the amplifier I0 and having a second input circuit coupled to the output circuit oi' the oscillator 20. The indicator 2I may include any suitable means, for example a nonlinear detector, for deriving a beat note from the oscillations of the amplifier I0 and those of oscillator 20 and includes a pair of headphones or a loudspeaker for audibly reproducing the beat note thus developed. Variations of the major com- 55 ponent of coupling may be indicated, if desired, by a suitable indicating device 22 which is coupled to an output circuit of amplifier I0 and is responsive to the amplitude of the oscillations produced by amplier I0. -50

As will presently be explained in greater detail, the amplifier Ib oscillates at a frequency such that the addition of all phase shifts through both the amplifier and the feed-back path is zero. The indicating system is provided with means included in the amplifier I0 for adjusting the amount of phase shift through the amplifier from the input terminals II to the output terminals I2. This adjustment is therefore eective to adjust the frequency of oscillation of the amplifier. This means is indicated schematically in Fig. 1 as a manually adjustable control or phase adjustor 23 and in practice may comprise an impedance network included in the translating channel of the amplifier I0 and having one or 75 more adjustable impedance elements which de termine the'phase shift' of oscillations translated by the network. 'I'he phase adjustor 23 may be providedwith a calibrated scale in order that it may be used to provide an indication of the change of phase angie needed to compensate for an object in the neighborhood of the search coll 5.

There is additionally included in the indicating system means responsive to the amplitude of oscillation of the amplifier III for automatically adjusting the magnitude of the major component of coupling provided by the feed-back path inversely with amplitude variation to maintain the oscillation amplitude of the amplifier I0 substantially constant. This means comprises an automatic volume control or A. V. C. rectifier 24 having an input circuit coupled to the output circuit of the amplifier I0 and having an output circuit which is coupled to the input circuit of one or more of the tubes of amplier I0 in conventional manner. While the amplitude of oscillation of the amplifier I0 may be limited merely by overloading, there is some advantage in providing the A. V. C. rectifier 24 particularly in that the amplitude of oscillation of the amplifier may be maintained within the range of its linear ampliflcation. It is desirable that the phase shift in the amplifier I0 be independent of the amplitude of its oscillation and the A. V. C..rectiiier 24 contributes to this result.

In order that the operativeness of the entire indicating system may be tested from time to time as desired, a single turn of wire 25 is flxedly positioned in concentric and coplanar relation with the inductors of the exploring unit I5 and is included in a series circuit comprising a resistor,

26 and a manually operable key or push button 21.

Considering now Vthe operation of the Fig. 1 arrangement, and referring to the graph of Fig. 2, the amplifier I0 has a positive' phase slope over its operating frequency range, as represented by curve A. Its phase slope must be substantial and in the same direction over the operating frequency range. as indicated by curve A. The smaller the phase slope, however, the greater the sensitivity of the indicating system as long as the phase slope meets the requirement that it be a controlling parameter, or "substantial," The useful range of phase angle is generally between i degrees and the phase slope is made small by securing this variation of phase angle over a wide range of frequency. It will be seen from Fig. 2 that a small phase shift Ao produces a relatively large change of frequency Af. A phaseshift characteristic of this nature is inherent in a broadly tuned amplifier; that is, an amplifier having a broad frequency-response characteristic of the type represented by curve B of Fig. 3. As previously stated, this characteristic is preferably provided by a single broadly resonant circuit provided in the amplifier I0.

The exploring unit I5 is so constructed that ind-uctors I8 and Ila-I1 normally have substantially no coupling. Hence, the only coupling between the output and input circuits of the amplier I0 is that provided by the reference impedance Il which, by virtue of the fact that its inductors I8 and I9 are coupled only inductively, provides a major component of coupling which is, therefore, a reactance component at all frequencies. This reactive component of coupling is made sufficiently large to cause the amplifier I0 to osclllate, the frequency of oscillation being that value which produces zero -phase shift of 7 oscillations translated from output terminals I2 of ampliner Il through the feed-back path including reference impedance i4 to the input terminals I I of the amplider and through the ampliiler I to its output terminals.

Assuming that the coupling between inductors la and I9 of reference impedance I4 has been adjusted to cause the amplifier I0 to oscillate strongly. the indicating system is initially adiusted for operation by adjustment of the phase adiustor 23 to provide a phase shift in adiustor 23 of approximately 9o degrees to compensate the 90-degree phase shift introduced by the reference impedance Il, whereby the frequency of oscillation of the amplifier IU is fn, Figs. 42 and 3. The reactance component of coupling provided by the reference impedance I4 is then adjusted to a value about twice that required to maintain the amplifier III in an oscillatory state. The oscillator 20 is then adiusted, if necessary, to the frequency of oscillation of amplifier Ill. With these simple adjustments completed, the indicating system is ready for operation.

If, now, the exploring unit I5 is brought into the vicinity of a metallic object. for example a buried metal object M. the coupling between its inductors I5 and Ils-I1 is disturbed and the inductors are no longer uncoupled. A conductive object, such as a metallic body. causes these inductors to have a large resistance component and a small reactance component of coupling. On the other hand, the magnetic material in ordinary dry ground causes the inductors to have mainly a small reactance component of coupling. The inductors I 6 and Ils-Ils of the exploring unit I5 are connected with such polarity in the indicating system that any reactance component of coupling between the inductors of the explor-- ing unit I5 merely adds to that provided by the reference impedance I4 and consequently affects only the amplitude of the oscillations produced by amplifier I0. The reactance component of exploring unit I5 is not sufdcient materially to change the amplitude of oscillation, however, due to the action of the A. V. C. rectifier 24 which operates to maintain the oscillation amplitude substantially constant.

Any resistance component of coupling between the inductors of exploring unit I5, on the other hand, has a quadrature relation to the reactance component of coupling and hence produces a change of phase of the oscillations translated through the feed-back path. When this phase change occurs, the phase shift of oscillations translated through both the feed-back path and the amplifier I0 no longer is zero. The amplifier III consequently changes its frequency of oscillation to a new value such that the phase shift oocurring in the amplifier itself at the new frequency, for example the phase shift A resulting from the change of frequency Af, Fig. 2, just compensates for the phase shift produced by the resistance component of coupling occurring in the exploring unit I5. The frequency of oscillation of the amplifier III is then no longer equal to that of the oscillator 2li and a beat note is consequently produced by the beat-note indicator 2| to provide an audible indication of the presence in the vicinity of the exploring unit I5 of a metallic body. for example the buried metal body M The phase adiustor 23 may now be so adjusted as to compensate for the change of phase shift of the oscillations translated through the feed-back path due to the presence of the conductive object M in the vicinityof the exploring unit I5. Ii.

this is done. the frequency of oscillation of the amplifier I0 returns to its initial value fn and the frequency of the beat note is reduced t0 zero. The necessary'change of phase shift of the phase adiustor 23 to produce this result is a measure of the magnitude of the resistance component of coupling produced between the conductors of the exploring unit Il by the conductive object M and, hence, provides a measure of the distance from the exploring unit I5 of a conductive body of known size and material. The phase adiustor 23 is preferably so calibrated over its range of adjustment. l

The relations between 'the reactance and resistance components of coupling occurring in the feed-back path are shown more clearly by the vector diagram of Fig. 4. The vector z represents the reactance component of coupling provided by the reference impedance I4 and any additional generally small reactance component of coupling .occurring in the exploring unit I5. The resistance component oi' coupling is in quadrature relation to the reactance component and is represented by the vector r, the resistance component being produced as previously explained by a conductive metallic body in the vicinity of the exploring unit I5. The total feed-back provided by the feed-back path is the vectorial sum of the reactance and resistance components and is represented by the vector R. The phase angle A between the resultant feedback when a resistance component is present and .the feedback without this component is the phase shift produced in the feed-back path by a conductive metallic body in the vicinity of the exploring unit I5 and is the phase shift for which the amplifier III compensates by a change of its frequency of oscillation. It will be evident from these vectorial relationships that, for greater sensitivity of the indicating system, it is desirable to make the reactance component as small as possible in comparison with the magnitude of the resistance component which will normally be encountered in the use of the system. Large amplification is desired in amplier I0, to cause it to oscillate with this small reactance component of coupling. Additionally, for any particular kind of conductive object there is an optimum frequency at which the resistance and reactance components produced in exploring unit I5 are approximately equal and resultant phase-angle change of the feedback is a maximum. Hence, it is preferable to operate the system at a frequency which provides the largest possible ratio of resistance component to reactance component to obtain optimum sensitivity.

Closure of the key "2l produces a resistance component of coupling in the exploring unit I5 and the resultant change of oscillation frequency of amplifier I0 provides an indication that the overall indicating system is operating properly.

The value of the resistor 26 may be so chosen that this change of frequency corresponds to the same change produced by a conductive object at a. known distance from the exploring unit I5. The change of frequency caused by a conductive body at an unknown distance from the exploring unit I5 may then be compared to the change of frequency caused upon closure of the key 21 after the exploring unit I5 has been moved out of the vicinity of the conductive body and these comparative indications are useful in estimating the distance or size of the conductive body from the the exploring unit I5 at the time the ilrst indication was obtained.

c nearer The mechanical construction of the exploring unit i5 may involve the use of metallic screws and metallic structural members which introduce an undesired-med r component of coupling between the inductors ofthe exploring unit i5. This coupling may be compensated by a compensator, not shown, having a pair of coupled inductors., One -of the compensator inductors is serially included in circuit with the inductors i6 and I8 and the second lnductor of the compensator is serially included in circuit with the induotors. i9 and lli-I1. The coupling between the inductors of the compensator is varied by an adjustable conductive memberlhaving a ratio of inductive reactance to resistance much less than unity. This member may, for example,

be a thin-walled brass cylinder. A compensator of this type is disclosed and claimed in the copending application ofLeslie F. Curtis, Serial No. 470,773, filed concurrently herewith, now United States Letters Patent No. 2,396,831, granted March 19, 1946.

Applications may arise where it is desirable to use the indicating system of the invention to measure the properties of dry ground, for example its magnetic properties, or to detect the presence of magnetic ore such as magnetite, or to locate an object exhibiting magnetic permeability which is submerged in salt water, the latter causing mainly a resistance component of coupling where the indicating system operates at a low audio-frequency. Fig. 5 is a circuit diagram of a portion of the indicating system shown in Fig. 1, similar circuit elements being designatedby similar reference numerals, and represents amodilied form of the invention suitable for this purpose. In this modification, the coupling impedance I3' comprises an adjustable resistor which is included in a circuit portion common to each of the inductors IB and lls--Ils of exploring unit i5 and provides primarily the major component of coupling effectiveto cause sustained oscillation of the amplifier l0. In this case, the major component ofcoupling is a resistance component and the phase adjuster 23 1S adjusted to have substantially zero phase shift at the .frequency fo. The operation of this modied form of the invention is essentially similar to that of Fig. 1, the system being initially adjusted for operation by adjusting the resistor 28 to a value about twice that necessary to produce sustained oscillation of the ampliier i andthe phase adjuster 23 being adjusted to cause the amplifier I0 to oscillate at the frequency fo where it causes no phase shift of the oscillations translated through the amplifier. As before,V the amplifier l0 oscillates at a frequency such that the phase shift of oscillations translated from the output terminals i2 of amplifier l0 through the feed-back path and through the amplier i0 to the output terminals is zero. The resistor 28 is now the reference impedance and determines which type of coupling component is effective to cause oscillations oi the ampliiler I0 and which type is effective to vary the frequency of oscillation. Since the resistor 28 provides primarily a resistance component of coupling, additional resistance components of coupling occurringin fthe exploring unit l are effective only to change the amplitude of oscillation of amplier l0, whereas reactance components of coupling occurring in the exploring unit I5 are effective to shift the frequency of oscillation. A body exhibiting magnetic permeability causes primarily a reactance component of coupling to occur between the inductors of 10 exploring unit il. Hence, this modiiied form oi the indicating system is particularly useful inlocating a. body which possesses magnetic permeability characteristics. For example. a body of iron submerged in salt water is readily located by an indicating system embodying this modified form of ,the invention, the salt water causing mainly a resistance component of coupling in the exploring unit Il thus affecting only the amplitude of oscillation of the amplier I0, whereas the body of iron produces a` reactance component of coupling in the exploring unit and consequently is effective to change the frequency of oscillation thus to provide an indication o! its proximity to the exploring unit IB. A vlow frequency is preferably employed to minimize the effect of the conductivity of the iron and water so that they do not tend unduly to change the amplitude oi oscillation of amplifier I 0.

The Figs. 1 and 5 arrangements embody the invention in perhaps its simplest forms. They have the limitation, however, that a certain amount of coupling produced in exploring unit i5 by external conditions may cause the amplifier I0 to stop oscillating if such coupling is suillciently large and of phase opposing that of the reference impedance i4. In this connection, it may be noted that it is possible to get such coupling in opposing phase because either polarity of coupling may result from objects in various positions relative to the exploring unit i5. This limitation is avoided in the modified form of the invention shown in Fig. 6 which is otherwise similar tothe Fig. 1 arrangement, similarV circuit elements beingl designated by similar reference numerals. It differs in that there is provided means responsive to the amplitude of oscillation cf the amplifier for automatically adjusting the magnitude of the major component of coupling, in this case the reactance component of coupling, inversely with the amplitude of oscillation to maintain the oscillation amplitude of the ampher iii-nearly constant. This means, in the present modification, comprises a bias-controlled repeater 29 included in the feed-back path and means for controlling the repeating ratio of the repeater-29 inversely in accordance with the oscillation amplitude. The repeater 29 comprises a pair of vacuum tubes 30, 3| having their input electrodes\ coupled in push-pull with a pair of resistors 32, 33, respectively, which are serially connected in the primary or output circuit of the amplifier i0. The output electrodes of tubes 30 and 3| are coupled in push-pull through a transformer 34 to the input circuit terminals il of the amplifier ill to modify the major or reactance component of coupling provided by the reference Vimpedance I4 and which is effective to cause sustained oscillation of the amplifier I0. A direct current meter 35 is included in the anode and screen electrode circuits of the tubes 30 and 3i and is used in a manner presently to be described initially to adjust the system for operation.

This modified form of the indicating system also includes a balanced rectifier 3B coupled to the output terminals i2 of the amplifier i0 to derive a control eilect or bias varying with the amplitude of oscillation of the amplifier and means for applying the control effect or bias in the same polarity to the input electrodes of the vacuum tubes 30 and 3l to maintain the amplitude of oscillation of the amplifier I0 substantially constant. The rectiiler 38 vcomprises a transformer 31 having a primary winding 3l coupled through a vacuum-tube repeater I9 to the output-circuit terminals I2 of the ampller I0. The transformer 31 has a center-tapped secondary winding 40; theend terminals of which are individually connected to the anodes of a4 pair of diode rectiiiers 4I, 42. A common load impedance is provided for the rectiers l4I and 42 and comprises two parallel resistor networks, one of which includes in series a resistor 43, a resistor 44 and the resistor 32, and the other network including in series a resistor 45, a resistor 49 and the resistor 33. The resistors 43 and 45 of this load network are connected in sexies between the control electrodes of vacuum tubes 30 and II while the resistors 44 and 48 are included in individual ones of the control-electrode circuits of these tubes.

The operation of this modification oi.' theinvention is essentially similar to that of Fig. 1 with the exception that in the present arrangement two reactivecomponents of coupling are applied through the single feed-back path from the output to the input circuit of the ampliiier IIL One reactive component of coupling is provided by the reference impedance I4 while the other is provided by the automatic amplitudecontrolsystem comprising the repeater 29 and the input circuit of the ampliiler I0, the repeater y 29 operating as a conventional push-pull amplifier. There is, however, a phase shift of 90 degrees of the oscillatory voltages translated through the repeater 29 due to the inductive coupling between the windings of the transformer 34 and the coupling provided by repeater 29 is thus a reactivecomponent. The transformer 34 is connected with such polarity that the reactive component provided by repeater 29 is in phase with, or adds to, that provided by the reference impedance I4 or, if desired, the coupling provided by the repeater 29 may be made more than enough to maintain sustained oscillation of amplifier I 0 and the reference impedance I4 is then so connected that its coupling opposes that of the repeater and is adjusted to a value which provides a suitable value of total coupling. 'I'he magnitude of the reactive component of coupling furnished by repeater 29 is controlled by a. bias voltage developed by the rectifier system 36 from the oscillations applied thereto through the repeater 39. 'Ihe magnitude of this bias voltage varies with the amplitude of the oscillations produced by amplifier I0 and the bias voltage is applied in the same polarity to the vacuum vtubes 30 and 3l to control their transconductance inversely with oscillation amplitude. Consequently, as the amplitude of the oscillations oi' amplifier I0 tends to increase, a larger bias 'voltage is developed by the rectier 36 and applied to the repeater 29v with the result that the magnitude of the reactive component of coupling provided by the repeater 29 is reduced. This reduced feedback causes the amplifier I0 to oscillate less strongly, thus reducing the amplitude of its oscillation. Should the oscillation amplitude tend to decrease, a control bias of less magnitude is developed by the reactier 36 with consequent increase ol the transconductance of the vacuum tubes and 2I and increase oi.' the reactive coinponent of coupling provided by the repeater 29. More feed-back energy is thus applied to the input circuit of the amplifier I0 and the amplitude oi' its oscillation consequently increases. The automatic amplitude-control system comprising repeater 29 and rectifier 99 thus tends to maintain Isubstantially constant the amplitude of oscillation of the amplifier III even though-the reactive component of coupling provided by the exploring unit I5 may change.

The indicating system of Fig. 8 is initially tubes, thus insuring that the repeater 29 normally operates somewhere near the middle of its transconductance range. Due to the action of the repeater 29 and reetiiler 2t, it will be found that the reference impedancev I4 has greatly reduced effect in controlling the amplitude of oscillation of the ampliiier III. On the other hand, it will also be found that the total reactance component of coupling. which is the sum of the reactance components provided by the repeater 29 and the reference impedance I4 is just suiiicient to maintain oscillation of the ampliner In and that this value of reactanee component is automatically maintained by the action oi' the repeater 29 and rectier 39. This small valueV of reactance component insures optimum sensitivity of the indicating system to resistance components oi.' coupling occurring in the exploring unit I5. The reference impedance I4 having once been adjusted, as described, does not need further attention over long periods of operation oi.' the indicating system. The only other initial adjustment which need be made to place the indicating system in condition for operation, and this adjustment may be necessary each time the indicating system is placed in operation, is the adjustment oi' the phase adjustor 23. This last-named adjustment is made in the-same The automatic amplitude-control system, com-4 prising the repeater 29 and rectifier 36 of the Fig. 6 modication of the invention, has the important advantage that it provides a coupling component of the same phase as that which normally maintains oscillations and the automatic bias developed by the rectiier 36 limits the amplitude of oscillation by compensating for any change in the total value of this component applied to the input circuit of amplifier I0. This insures that the ampliier I0 shall be maintained in continuous oscillation for any reasonable amount of change of coupling arising from causes external to the indicating system.

The only changes necessary to adapt the Fig. 6 arrangement to be responsive to bodies exhibiting magnetic permeability, that is to reactance components of coupling produced in the exploring asedio? It is occasionally desirable to bury 'a non-conductive-nonmsgnetic object at a spot which can'- not be conveniently marked but which must be accurately located at some future time. The indicating system of the present invention is particularly suited for this purpose. Fig. "i represents a novel application of this nature. Here a nonconductive-nonmagnetic object B is buried and with it is buried a fixed-tuned circuit I1 sharply resonant at a frequency within the operating frequency range of the system. A similarly buried object B' also has buried with it a fixed-tuned circuit Il sharply resonant at the same or a different frequency. Assume. for example. that the circuit 41 is tuned to 100 kilocycles and the circuit Il to 110 kilocycles. The indicating system of the invention, which may be of either the Fig. 1 or Fig. 6 types and is indicated in Fig. '7 in schematic manner as` 48, is adjusted for operation at the'frequen'c'y of the buried tuned circuit associated with the object to be located. Under the conditions assumed, if the indicating system is adjusted for operation at 100 kilocycles and the exploring unit i! is moved into the vicinity of the tuned circuit 41, the impedance of the latter at this frequency is predominantly resistive and it, therefore, introduces a resistance component .of coupling in the exploring unit of the system, thus to provide.

an indication that the tuned circuit 41 is within the vicinity of the exploring unit. However, should the exploring `unit I5 of the indicating system be moved, as indicated in broken lines, into the vicinity of the tuned circuit 4.8 asso ciated with the buried object B', the impedance of the tuned circuit I0 is predominantly reactive at 110 kilocycles and consequently only a reactance component of coupling is produced in the exploring unit i5 by this tuned circuit. Only the amplitude of oscillation of the system is thus affected and no resultant 'indication is provided. It may be noted in this regard that the tuned circuit 48 has a predominantly reactance component of impedance at any frequency which differs appreciably from its resonant frequency. Thus, an element of secrecy is introduced in that the buried object cannot readily be located, even with the indicating system of the invention. unless the resonant frequency of the buried tuned circuit is known.

The sensitivity of either the Fig. 1 or Fig. 6 arrangement can be increased by using a frequency-multiplier stage between the output circuit of the amplifier i0 and the input circuit of the beat-note indicator 2i toprovide from the oscillations of amplifier l0 other oscillations of higher frequency. The operating frequency of the oscillator is, of course, increased by the same amount. A small change ofthe frequency of oscillation of amplifier I0 then produces a much larger change of frequency of the oscillations applied to the indicator 2i to cause a beat note of'hlgher frequency to be developed between these oscillations and the oscillations of oscillator 20.

While the exploring unit IB is preferably so constructed that its inductors i0 and Ils-ils normally have substantially no coupling and the major component of coupling necessary to main tain oscillation of amplifier Il is provided by the reference 'impedance i4 in Figs. 1 and 6, or the reference'impedance comprising the coupling resistor 28 in Fig. 5, it will be understood that the maior component of vcoupling may be provided primarily by a small coupling initially provided between the inductors of the exploring unit VII bysuitable design 0! the latter. In this 088e,

the reference impedance may be omitted from the system. Optimum sensitivity of the system, however, requires a' carefully controlled value of the major component of coupling and this is most readily and easily eifected by the use of the reference impedance. a

As illustrative of specific embodiments of the invention, the following circuit constants are given for the embodiments of the invention of the types shown in Figs. 1 and 8:

Mutual inductance of reference impedance M---. :0.1 microhenry Exploringunit Il: g l

Inductor Il- Diameter "1% inches Number of turns.....v 80 Inductance 2.8 millihenries Resistance 4 ohms Inductorsection il.- 1| i i l Diameter 4H inches Number of turns---- 80 Inductance 1.9 millihenries Inductor section I1- [PIl l lf@ l Diameter 12 inches Number of turns-..-- 50 Inductance 2.3 milllhenries Inductor sections Il. and

'Iotal inductance---- 3.4 millihenries Total resistance 8 ohms Resistor 26.....--........ 100ohms Resistors 43, 4I, 4I, 4G, each-.. 0.1 megohm Resistors l2, 33, each 1 ohm 'I ubes I0, Il lNcGT Type Transformer 34 Air-core windings of fine wire, paper layer construction Operating frequency:

A=0 1000 cycles A=i90 degrees 1000x350 cycles While there have been-described what are at present considered to be the preferred embodi- 'flications may be made therein without departing from the invention. and it is, therefore, aimed in the appended claims to cover all such changes and modications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A variable-frequency oscillatory system comprising, a stable ampline having input and output terminals and having substantial ampliflcation and a positive phase slope over a predetermined frequency range. a single feed-back path substantially nonselective over said range coupling said output terminals to said input terminals, means included in said feed-back path for establishing a feed-back component. of coupling of a type eective primarily to cause sustained oscillation of said amplifier at a frequency in said range but ineffective to cause substantial variawith variations of said component, and means included in said feed-back path for establishing a second component of feed-back coupling substantially independent of and-in quadrature relation to said first-mentioned component, said second component being variable in response to a frequency-control condition external to said lastmentioned means and being effective primarily to cause variations of the frequency of oscillation in accordance with variations of said condition. e.

2. A variable-frequency oscillatory system comprising. a stable amplifier having input and output terminals and having substantial ampliilcation and a relatively small positive phase vslope over a predetermined frequency range, a single 1 feed-back path substantially nonselective over said range coupling said output terminals to said input terminals. means included in said feedback path for establishing a feed-back component of coupling of a type effective primarily to cause 2 response to a frequency-control condition external to said last-mentioned means and being eective primarily to cause variations of the frequency of oscillation in accordance with variations of said condition.

3. A variable-frequency oscillatory system comprising, a stable amplier having input and output terminals and having over a predetermined frequency range substantial amplification and a positive phase slope the values of which are substantially independent of the amplitude of oscillations translated by said amplifier, a single feed-back path substantially nonselective over said range coupling said output terminals to said input terminals, means included in said feed-back path for establishing a feed-back component of coupling of a type effective primarily to cause sustained oscillation of said amplifier at a frequency in said range but ineffective to cause substantial variations of the oscillation frequency of said amplier withvariations of said component,

and means included in said feed-back path for establishing a second component of feed-back coupling substantially independent of and in quadrature relation to said first-mentioned component, said second component being variable in response to a frequency-control condition external to said last-mentioned means and being effective primarily to cause variations of the frequency of oscillation in accordance with variations of said condition.

4. A variable-frequency oscillatory system comprising, a stable amplifier having input and output terminals and a single selective circuit broadly resonant over a predetermined frequency range and having substantial amplication, a

single feed-back path substantially nonselective over said range coupling said output terminals to said input terminals, means included in said feedback path for establishing a feed-back component of coupling of a type effective primarily to cause sustained oscillation of said amplifier at a frequency in said range but ineffective to cause substantial variations of the oscillation frequency of said amplifier with variations of said component, and means included in said feed-back path for establishing a second component of feed-back coupling substantially independent of and in quadrature relation to said first-mentioned component, said second component being variable in response to a frequency-control condition external to said last-mentioned means and being effective primarily to cause variations of the frequency of oscillation in accordance -with variations of said condition.

5. A variable-frequency oscillatory system comprising, a stable amplier having input and output terminals and having substantial ampliilcation and a positive phase slope over a predetermined frequency range, a single feed-back path substantially nonselective over said range coupling said output terminals to said input terminals, means included in said feed-back path for establishing one of a reactance and resistance component of feed-back coupling eective primarily to cause sustained oscillation of said amplifier at a frequency in said range but ineffective to cause substantial variations of the oscillation frequency of said amplifier with variations of said one component, and means included in said feed- 25 back path for establishing the other of said reactance and resistance components, said other component being variable in response to a frequency-control condition external to said lastmentioned means and being effective primarily to cause variations of the frequency of oscillation in accordance with variations oi' said condition.

6. A variable-frequency oscillatory system comprising, a stable amplifier having input and output terminals and having substantial ampliilcation and a positive phase slope over a pre-,

determined frequency range, a single feed-back path substantially nonselective over said range coupling said output terminals to said input terminals, means included in' said feed-back path for establishing a reactance component of feedback coupling effective primarily to cause sustained oscillation of said amplifier at a frequency in said range, and means in said feed-backipath for establishing a. resistance component of feedback coupling variable in response to a frequencycontrol condition external to said last-mentioned means` and effective primarily to cause variations of the frequency of oscillation in accordance with variations of said condition, variations of said reactance component of coupling being ineffective in the absence of said resistance component of coupling to cause substantial variations of the frequency of oscillation of said amplifier.

7. A variable-frequency oscillatory system comprising, a stable amplifier havingv input and output terminals and having substantial ampliication and a positive phase slope over a predetermined frequency range, a single feed-back path substantially nonselective over said range coupling said output terminals to said input terminals, means included in said feed-back path for establishing a resistance component of feedback coupling effective primarily to cause sustained oscillation of said amplifier at a frequency in said range, and means in said feed-back path for establishing a reactance component of feedback coupling variable in response to a frequencycontrol condition external to said last-mentioned means and effective primarily to cause variations of the frequency of oscillation in accordance with variations of said condition, variations of said resistance component of coupling being ineffective in the absence of said reactance component of coupling to cause substantial vari- 17 ations of the frequency of oscillation of said amplifier.

8. A variable-frequency oscillatory system comprising, a stable amplifier having input and output terminals and having substantial amplification and a positive phase slope over a predetermined frequency range, a single feed-back path substantially nonselective over said range coupling said output terminals to said input terminals, a coupling impedance included in said feed-back path for establishing a feed-back component of coupling of a type effective primarily to cause sustained oscillation of said amplifier at a frequency in said range but ineffective to cause substantial variations of the oscillation frequency of said amplifier with variations of said component, and a second coupling impedance included in said feed-back path for establishing a second component of feed-back coupling independent of and in quadrature relation to said first-mentioned component, said second component being variable in response to a frequency-control condition external to said second coupling impedance and being effective primarily to cause variations of the frequency of oscillation in accordance with variations of said condition.

9. A variable-frequency oscillatory system comprising. a stable amplifier having input and output terminals and having substantial ampliiication and a positive phase slope over a predetermined frequency range. a single feed-back path substantially nonselective over said range and coupling said output terminals to said input terminals. and a coupling impedance included in said feed-back path and having two independent impedance portions which provide individual ones of quadrature-related components of feed-back coupling of which one component is effective primarily to cause sustained oscillation of said amplifier at a frequency in said range and variations of said one component in the absence of the other component are ineffective to cause substantial variations of the oscillation frequency of said amplifier while the other component is variable in response to a frequency-control condition external to said coupling impedance and is effective primarily to cause variations of the frequency of oscillation in accordance with variations of said condition.

10. A variable-frequency oscillatory system comprising, a stable amplifier having input and output terminals and having substantial amplification and a positive phase slope over a predetermined frequency range, a single feed-back path substantially nonselective over said range coupling said output terminals to said input terminals. and means included in said feed-back path for independently establishing quadraturerelated components of feed-back coupling of which one component is effective primarily to cause sustained oscillation of said amplifier at a frequency in said range and variations of said one component in the absence of the other component are ineffective to cause substantial variations of the oscillation frequency of said ampliiler while the other component is variable in response to a frequency-control condition external to said last-mentioned means and is eiective primarily to cause variations of the frequency of oscillation iu accordance with variations of said condition, said last-mentioned means comprising an auxiliary reference impedance providing primarily said one component of coupling and a pair of inductors normally naving substantially no coupling but becoming cou- 18 pled in response to said condition to provide primarily said other component oi coupling.

11. A variable-frequency oscillatory system comprising, a stable amplifier having input and A output terminals and having substantial ampliication and a positive phase slope over a predetermined frequency range; a single feed-back path substantially nonselective over said range coupling said output terminals to said input terminals. and'means included in said feed-back path for independently establishing quadrature- 'related components of feed-back coupling of which one component is effective primarily to cause sustained oscillation of said amplifier at a frequency in said range and variations of said one component in the absence of the other component are ineffective to cause substantial variations of the oscillation frequency of said ampliner while the other component is variable in response to a frequency-control condition external to said last-mentioned means and is eective primarily to cause variations of the frequency of oscillation in accordance with variations of said condition, said last-mentioned means c0mprising a first pair of coupled inductors providing primarily said one component of coupling and a second pair of inductors normally having substantially no coupling but becoming coupled in response to said condition to provide primarily said other component of coupling.

l2. A variable-frequency oscillatory system comprising, a stable amplifier having input and output terminals and having substantial ampliilcation and a positive phase slope over a predetermined frequency range. a single feed-back path substantially nonselective'over said range v coupling said output terminals to said input terminals, means included in said feed-back path for establishing a feed-back component of coupling of a type effective primarily to cause sustained oscillation of said amplifier at a frequency in said range but ineffective to cause substantial variations of the oscillation frequency of said amplifier with variations of said component, and means included in said feed-back path for establishing a second component of feed-back coupling substantially independent of and in quadrature relation to said first-mentioned component, said second component being variable in response to a frequency-control condition external to said last-mentioned means and being eiective primarily to cause variations of the frequency of oscillation in accordance with variations of said condition, and means included in said amplifier for adjusting the phase shift of oscillations translated therethrough from said input to said output terminals to adjust the frequency of oscillation oi' said amplifier.

13. A variable-frequency oscillatory system comprising, a stable amplifier having input and output terminals and having substantial ampliflcation and a positive phase slope over a predetermined frequency range, a single feed-back path substantially nonselective over said range coupling said output terminals to said input terminals. and means included in said feed-back path for independently establishing quadraturerelated components of feed-back coupling of which one component is effective primarily to cause sustained oscillation of said amplifier at a frequency in said range and variations of said one component in the absence of the other component are ineffective to cause substantial variations of the oscillation frequency of said ampliiler while the other component is variable in usare? response to a frequency-control condition external to said last-mentioned means and is eircotive primarily to cause variations of the frequency'oi' oscillation in accordance with variations of said condition. said last-mentioned means comprising a pair of inductors normally having substantially no coupling but becoming coupled in response to said condition to provide primarily said other component of coupling and a resistor included in a circuit portion common to each of said inductors and providing primarily said .one component of coupling.

14. A variable-frequency oscillatory system comprising, a stable amplier having input and output terminals andhaving substantial ampliilcation and a positive phase slope over a predetermined frequency range. a single feed-back path substantially nonselective over said range coupling said output terminals to said input terminals, means included in said feed-back path for independently establishing quadrature-related components of feed-back coupling of which one component is eii'ective primarily to cause sustained oscillation of said amplifier at a frequency in said range and variations of said one component in the absence of the other component are ineffective tol cause substantial variations of the oscillation frequency of said ampli- .iler while the other component is variable in response to a frequency-control condition external to said last-mentioned means and is effective primarily to cause variations of the frequency of oscillation in accordance with variations oi said condition. and means responsive to the amplitude of oscillation of said ampliiler for automatically maintaining the oscillation amplitude thereof substantially constant.

15. A variable-frequency oscillatory system comprising, a stable ampliiier having input and output terminals and having substantial ampliilcation and a positive phase slope over a predetermined frequency range, a single feed-back path substantially nonselectiveover said range coupling said output terminals to said input terminals, means included in said feed-back path for independently establishing quadrature-rclated components of feed-back coupling of which one component is effective primarily to cause sustained oscillation of said amplifier at a frequency in said range and variations of said one component in the absence of the other component are ineiective to cause substantial variations of the oscillation frequency of said ampliiler while the other component is variable in response to a frequency-control condition external to said last-mentioned means and is etfective primarily to cause variations of the frequency of oscillation in accordance withvariations of said condition, and means responsive to the amplitude of oscillation of said amplifier for automatically adjusting the magnitude of said one component of coupling inversely therewith to maintain the oscillation amplitude of said amplifier substantially constant.

16. A variable-frequency oscillatory system comprising, a stable amplifier having input and output terminals and having substantial ampliilcation and a positive phase slope over a predetermined frequency range, a single feed-back path substantially nonselective over said range coupling said output terminals to said input terminals. means included in said feed-back path for independently establishing quadrature-related components of feed-back coupling of which sustained oscillation oi said amplier at a frequency in said range and variations oi said one component in the absence oi the other component are ineifective to cause substantial variations ofthe oscillation frequency'of said ampliiler while the other component is variable in response to a frequency-control condition external to said last-mentioned means and is eilective primarily to cause variations of the frequency of oscillation in accordance with variations of said condition, said last-mentioned means including a vacuum-tube repeater 'in said feed-back path to provide said one component of coupling. and means for controlling the repeating ratio oi' said repeater to control the amplitude oi oscillation of said amplifier.

17. A variable-frequency oscillatory system comprising. a stable amplier having input and output terminals and having substantial ampliiication and a positive phase slope over a predetermined frequency range, a single feed-back path substantially nonselective over said range coupling said output terminals to said input terminals. means included in said feed-back path for independently establishing quadrature-related components of feed-back coupling of which one component is eiective primarily to cause sustained oscillation of said amplifier at airequency in said range and variations of said one component in the absence of the other component are ineffective to cause substantial variations of the oscillation frequency of said ampliiler while the other component is variable in response. to a frequency-control condition external to said last-mentioned means and is effective primarily to cause variations of the frequency of oscillation in accordance with variations of said condition, said last-mentioned means including a bias-controlled repeater in said feed-back path to provide said one component of coupling, and means for deriving a control biasvarying with the amplitude of oscillation and for utilizing said bias to control the repeating ratio of said repeater inversely with oscillation amplitude to maintain the oscillation amplitude of said ampliher substantially constant.

18. A variable-frequency oscillatory system comprising, a stable amplier having input and output terminals and having substantial ampliilcation and a positive phase slope over a predetermined frequency range. a single feed-back path substantially nonselective over said range coupling said output terminals to said input terminals, means included in said feed-back path for independently establishing quadrature-related components of feed-back coupling of which one component is eiective primarily to cause sustained oscillation of said amplifier at a frequency in said range and variations of said one component in the absence of the other component are ineffective to cause substantial variations of the oscillationfrequency of said ampliiler while the other component is variable in response to a frequency-control condition external to said lastmentioned means and is eiective primarily to cause variations of the frequency of oscillation in accordance with variations of said condition, said last-mentioned means including a pair 0f vacuum tubes having input electrodes coupled in push-pull to said output terminals and having output electrodes coupled in push-pull to said input terminals to provide said one component of coupling, rectifier means coupled to said output terminals to derive a control effect varying one component is effective primarily to cause l5 with the amplitude of oscillation of said ampli- 21 ner. and means for applying said control eifect in the same polarity to the input electrodes ot said vacuum tubes to maintain the amplitude of oscillation of said amplifier substantially constant.

19. A variable-frequency oscillatory system comprising, a stable amplier having input and output terminals and having substantial ampliflcation and a positive phase slope over a predetermined frequency range, a single feed-back path substantially nonselective over said range coupling said output terminals to said input terminals, means included in said feed-back path for independently establishing quadrature-related components of feed-back coupling of which one component is eiective primarily to cause sustained oscillation of said amplifier at a frequency in said range and variations of said one component in the absence of the other component are ineffective to cause substantial variations of the oscillation frequency of said amplifier while the other component is variable in response to a frequency-control condition external to said lastmentioned means and is eiective primarily to cause variations of the frequency of oscillation in accordance with variations of said condition, said last-mentioned means including a pair of vacuum tubes having input electrodes coupled in push-pull to said output terminals and having output electrodes coupled in push-pull to 30 said input terminals to provide said one component of coupling, a balanced rectiiier coupled to said output terminals to derive a control effect varying with the amplitude of oscillation of said amplier. and means for applying said control effect in the same polarity to the input electrodes of said vacuum tubes to maintain the amplitude of oscillation of said amplifier substantially constant.

` mom a WHEELER,

REFERENCES crrnn The following references are of record in the file of this Patent:

UNITED STATES PATENTS Number Name Date 1,984,465 Dana Dec. 18. 1934 2,018,080 Martienssen Oct. 22, 1935 2,027,291 Robinson Jan. 7, 1936 2,048,591 Berry July 21. 1938 2,146,073 Jennens et al. Feb. 7, 1939 2,220,070 Aiken -..'Nov. 5, 1940 2,222,182 Mounce et al. Nov. 19, 1940 2,267,884 Zuschlog -i Dec. 30, 1941 2,271,951 Pearson et al. Feb. 3, 1942 2,364,957 Douglas Dec. 12, 1944 OTHER REFERENCES Terman et al., Proceedings of the 'Institute of Radio Engineers, pages 649-655, Oct. 1939. 

